1. Field of the Invention
The present invention relates to a water content estimation apparatus and a water content estimation method for estimating a water content of a sheet material, specifically to an apparatus and a method for estimating an accurate water content of the sheet material when the sheet material is actually processed, by correcting a water content detected before the sheet material is processed; and to a sheet material processing apparatus including the water content estimation apparatus, and a sheet material processing method including the water content estimation method.
2. Description of the Related Art
In an image formation apparatus using an electrophotographic technology, a water content of a sheet material on which image formation is performed greatly affects an image quality and processing stability. This is because when the water content increases, resistivity of the sheet material decreases, thereby deteriorating transferring performance of a toner image and separation performance thereof from a photosensitive drum. In a fixing processing in which heating and pressing are performed, due to heat of evaporation of moisture contained in the sheet material, there is caused fluctuation in effective fixing temperature.
In recent years, in a sheet material processing apparatus as represented by an image formation apparatus (such as LBP or copying machine), with increasing demand for higher image quality and higher processing speed, optimization control of processing conditions complying with each of various states of the sheet material has been performed.
One of the most important sheet materials processed by the sheet material processing apparatus is paper. Mechanical and electrical characteristics of paper greatly fluctuate according to its water content. Therefore, as information on the sheet material, the water content is particularly important.
Conventionally, an attempt is made to control the processing conditions for the sheet material by measuring the water content of the sheet material in the sheet material processing apparatus. Japanese Patent Application Laid-Open No. H07-234556 discloses a water content detection apparatus, which applies projection light to an object to be measured, for measuring the thickness and water content of the sheet material from a reflection light amount. In this case, the water content is measured based on humidity of the object to be measured and on a distance from a humidity detection unit to the object to be measured. Based on a result of comparison between the measured water content and a predetermined value, the water content of the object to be measured is computed. Then, based on the computation result, various image forming conditions are corrected. Alternatively, based on a distance between a humidity detection unit and a recording sheet (sheet material), the water content of the object to be measured is computed.
However, paper which is a sheet material mainly used in the image formation apparatus or the like is significantly changed in various characteristics by entering and releasing of moisture. The water content rapidly fluctuates particularly during a time until the sheet material reaches an equilibrium state with ambient environment (particularly, temperature and humidity). This time corresponds to a time immediately after the sheet material is taken out from a package having high air tightness, a time immediately after the sheet material is dried by heating in a fixing step by a copying machine or the like, a time immediately after image formation using ink by an ink jet printer, or the like. Accordingly, there is a problem in that, even when the water content is detected, a value fluctuates after a time required for conveying or temporarily storing the sheet material has elapsed after the detection, so that various set conditions for handling the sheet materials deviate from an appropriate range. This is a significant problem particularly in image formation by a printer for successively performing a plurality of processings such as transfer, fixation, and the like of coloring materials while conveying the sheet material.
For this problem, there is proposed to provide water content sensors in a storage unit and a conveying path for the sheet material to detect the water content of the sheet material and reflect the detection result to a charging voltage, a separation voltage, a fixing temperature, and the like.
Japanese Patent Application Laid-Open No. H07-234556 discloses an image formation apparatus including water content sensors, which are provided in a storage unit for the sheet material and a reverse conveying path for two-side printing, for detecting an infrared absorption peak. The water content of the sheet material detected by the water content sensor is reflected to a charging voltage and a separation voltage of the image formation apparatus. The water content which has changed due to a fixing processing on one surface is detected by the water content sensor on the reverse conveying path, and the charging voltage and the separation voltage are optimized for image formation on the other side.
Japanese Patent Application Laid-Open No. H11-202686 discloses an image formation apparatus in which humidity in a sheet material conveying space is detected to estimate a water content of a sheet material, and processing conditions for image formation are adjusted based on the estimated water content. In this case, moisture contained in the sheet material is forcibly allowed to evaporate by using heat of a fixing roller, humidity in the sheet material conveying space which changes according to an amount of water vapor generated thereby is detected, and the water content of the sheet material is estimated from the humidity. The detection of humidity is performed at a predetermined time after the sheet material passes the fixing roller. Therefore, the water content can be estimated separately for each sheet material. Accordingly, processing conditions can be adjusted following the water content of the sheet material, which fluctuates from moment to moment.
Japanese Patent Application Laid-Open No. H05-204411 discloses a sheet feed apparatus in which fuzzy inference is executed based on humidity inside the sheet feed apparatus and a time derivative value of the humidity, thereby performing an ON/OFF control of a heater.
A general sheet material is thin and moisture enters and is released from a surface thereof due to a humidity difference with respect to environment. Therefore, there is a possibility of the water content of the sheet material changing according to temperature and humidity of a space where the sheet material is conveyed and processed, and elapsed time therefor.
In the image formation apparatus disclosed in Japanese Patent Application Laid-Open No. H07-234556, the water content of a sheet material is detected in a state where sheet materials are stacked in the storage unit. Processing conditions for image formation are set according to the water content of the sheet material in the state where the sheet materials are stacked in the storage unit.
Accordingly, after the sheet material is taken out from the storage unit, the sheet material is conveyed in the image formation apparatus and waits on the conveying path, until a moment at which the sheet material is actually subjected to image formation, there is a possibility of the water content having been changed. In this case, image formation is performed on the sheet material under processing conditions improper for the sheet material changed in water content.
For example, when, with respect to the sheet material increased in water content by being exposed to high humidity in the image formation apparatus, the processing is performed with a transferring bias voltage and a separation bias voltage being optimized according to the water content of the dried sheet material detected in the storage unit, there may be a risk of interfering transfer of a toner image and separation of the sheet materials. In other words, at a fixing temperature optimum for the dried sheet material, the toner image cannot be sufficiently fixed to the sheet material increased in water content.
In the image formation apparatus disclosed in Japanese Patent Application Laid-Open No. H11-202686, the water content of the sheet material on which image formation is completed (image is fixed) is discriminated. Therefore, the discriminated water content does not necessarily match with a water content of the sheet material to be subjected to image formation. When a water content of a first sheet and a water content of a second sheet material are different from each other, processing conditions set according to the water content of the first sheet material is inappropriate for the second sheet material.
In an image formation apparatus disclosed in Japanese Patent Application Laid-Open No. H05-204411, a water content in an atmosphere inside the image formation apparatus can be predicted accurately to some extent. However, the water content of the sheet material itself is not detected. Therefore, when the water content is different among the sheet materials, processing conditions cannot be controlled appropriately in some cases. In general, the sheet materials are accommodated in the image formation apparatus in a state where several tens to several hundreds of sheet materials are stacked on one another. Water contents of the sheet materials differ from one another according to a position of each sheet material in the stacked sheet materials. Specifically, an uppermost sheet material has a surface exposed to an atmosphere, so that the equilibrium state of the water content thereof is easily achieved, and the water content is close to a water content of the atmosphere where the sheet material is stored. On the other hand, the water content of the sheet material in the stacked sheet materials is different from the water content of the atmosphere because the sheet material is sandwiched between the other sheet materials and movement of moisture thereof with respect to the atmosphere is limited. For example, the first sheet material and the tenth sheet material are different from each other in water content. Particularly in a case immediately after sheet materials are newly supplied, or the like, a water content of the sheet material and the water content of the atmosphere inside the image formation apparatus are not equilibrated, and the water contents of the sheet materials are considerably different from one another. This is because it takes substantially long time for achieving an equilibrium state between water contents of the sheet material and humidified atmosphere in the image formation apparatus. Therefore, the water content contained in the sheet material cannot be unambiguously determined only from the water content of the atmosphere inside the image formation apparatus. As a result, even when the water content of the atmosphere can accurately be predicted, processing conditions may not be controlled appropriately in some cases.